well foxy, most of them are same speed since its the same chip & memory at same clock speeds.
There are these "Golden Sample" or "Deluxe" or "Extreme" kind of editions (just OCed a bit by default) some manufacturers like GainWard or Leadtek offer, but they usually cost a bit more than the standard model of a card and IMO the performance gain isnt big enough to justify that (unless its a X800Pro @ XT mod or something, i.e. with extra-pipelines by default n that).
The only other big difference between the various manufacturers is the cooling solution on the card - most of them just stick to the nVidia reference design but put their fancy colorful logo on it to make the plastic look 1337, but that of course doesnt make it any better. Brands like Leadtek are known to always develop their own cooling solution, which may be really huge (like on the GF6800 series) but at the same time is also more effective and quiet than the reference design.
And the software-bundles that come with each card also vary a good bit (no games / few games / few games + apps / etc.)
In the end you'll have to check reviews about which brand suits you best and is good at the above criteria.